A device for monitoring doors of an elevator installation is known from U.S. Pat. No. 6,193,019 B1, wherein the doors are present on several floors. In that case, a safety circuit in the form of a chain of switches, which are arranged at locks of the doors, is present. A resistance is connected in parallel with each of these switches. The respective resistance is bridged over when the switch is closed. The resistance values are formed as a product of a power of two and a reference resistance. The exponent, which is a non-negative whole number, in that case corresponds with the floor on which is arranged the door to which the lock with the associated resistance relates. If one or more switches are opened then it can be calculated from the sum of the resistances at the open switches, which indicates the total resistance with respect thereto of the resistances at the open switches, which switches are open. This can be output by way of a suitable indicating device.
The device, which is known from U.S. Pat. No. 6,193,019 B1, for monitoring the doors of an elevator installation has the disadvantage that opening and subsequent closing of a lock cannot be recognized if the energy supply is interrupted during this time period.
A door closure for a door of an elevator is known from EP 1 440 930 A2. The known door closure comprises a housing and a latch, wherein the latch is transferrable with respect to the door into a locking setting and into a release setting. The latch is in that regard mounted in the housing. The closure comprises a manually actuable emergency unlocking device for transferring the latch into its release setting, wherein the emergency unlocking device comprises an entrainer which when the latch is transferred into the release setting is actuable by an emergency unlocking key. In addition, a constrainedly opening safety switch or at least a part of a corresponding electrical safety circuit and a manually actuable entrainer associated therewith for actuation of the safety switch or the safety circuit is present at or in the housing. The door closure can be constructed in such a manner that through actuation of the emergency unlocking device initially the safety switch is actuated and subsequently thereto the latch is transferred into its release setting. Normal operation of an item of equipment associated with the door can be interrupted and reinstated by a safety circuit when the safety switch is switched.
In the case of, especially, elevator installations, which have a reduced shaft head or no shaft head in the elevator shaft, monitoring of door closures, particularly three-square head locks, of shaft doors is required. It can thereby be established whether anybody could have gone onto the car roof of the elevator car. When a three-square head lock is opened a safety switch is opened which interrupts a safety circuit. If the safety switch, as described in EP 1 440 930 A2, has a detent function then the elevator installation is stopped until a service engineer places this back in operation after a suitable check. However, if, for example, an energy supply now temporarily fails then opening and subsequent closing of the switch is not recognized. If this problem is not taken into account then somebody can go unrecognized into the elevator shaft, particularly onto the car roof without this being recognized, as a result of which serious accidents can happen. In order to take this into account it is conceivable that, after power failure, placing of the elevator installation into operation may be carried out only after a prescribed taking-out-of-operation program by an authorized service engineer. However, this always has to be carried out regardless of whether or not anybody has opened a door. This has the consequence, even in the case of comparatively short failures of the energy supply, of a substantial disruption to the operating program, since the elevator installation is out of operation for a comparatively lengthy period of time determined by, inter alia, the time until arrival of the service engineer and the time for carrying out the taking-out-of-operation program.